The Hydraulic Chain Saw is typically used to avoid over cuts. When cutting a scupper hole for example or a mechanical opening in a wall or slab that cannot be over cut.
Chain Saws are used when enlarging or creating door openings. The process usually starts with the Hand Saw to make the bigger opening, then you would use the chain saw to finish the corners.
The bars on the Chain Saw vary in size from 14 inches to 3 ft long. Unless you really need it you try to avoid using the chain saw. But the Chain Saw is a very valuable tool.